Lignin is the largest source of renewable aromatic compounds on the planet. In recent years, lignin has received increased attention in academia and industry for its potential to be converted into commodity chemicals. Whereas lignin has historically been burned to provide energy, new efforts have focused on utilizing and manipulating its inherent structure and functionality. See Boerjan, W.; Ralph, J.; Baucher, M., Lignin biosynthesis. Annual Reviews in Plant Biology 2003, 54, 519-546; Li, C. Z.; Zhao, X. C.; Wang, A. Q.; Huber, G. W.; Zhang, T., Catalytic transformation of lignin for the production of chemicals and fuels. Chemical Reviews 2015, 115, 11559-11624; Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M., The catalytic valorization of lignin for the production of renewable chemicals. Chemical Reviews 2010, 110, 3552-3599; Rinaldi, R.; Jastrzebshi, R.; Clough, M. T.; Ralph, J.; Kennema, M.; Bruijnincx, P. C. A.; Weckhuysen, B. M., Paving the way for lignin valorisation: Recent advances in bioengineering, biorefining and catalysis. Angewandte Chemie (International Edition) 2016, 55, 8164-8215; Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E., Lignin valorization: Improving lignin processing in the biorefinery. Science 2014, 344, 1246843; and Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T., The path forward for biofuels and biomaterials. Science 2006, 311, 484-489. For example, various strategies have shown promise in deriving valuable chemicals from the lignin portion of biomass feedstocks, including lignin catalytic cracking, hydrolysis, reduction/hydrogenolysis, and oxidation. See Zakzeski, J.; Jongerius, A. L.; Bruijnincx, P. C.; Weckhuysen, B. M., Catalytic lignin valorization process for the production of aromatic chemicals and hydrogen. Chem Sus Chem 2012, 5, 1602-1609; Shuai, L.; Amiri, M. T.; Questell-Santiago, Y. M.; Héroguel, F.; Li, Y.; Kim, H.; Meilan, R.; Chapple, C.; Ralph, J.; Luterbacher, J. S., Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization. Science 2016, 354, 329-333; and Badalyan, A.; Stahl, S. S., Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators. Nature 2016, 535, 406-410. Of these methods, oxidative treatments present several advantages and have the potential to yield more highly functionalized monomers or oligomers, which could be useful within the chemical industry. See, for example, Ma, R.; Xu, Y.; Zhang, X., Catalytic oxidation of biorefinery lignin to value-added chemicals to support sustainable biofuel production. Chem Sus Chem 2015, 8, 24-51; Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N.J., Isolation of functionalized phenolic monomers through selective oxidation and C—O bond cleavage of the β-O-4 linkages in lignin. Angewandte Chemie (International Edition) 2015, 54, 258-262; Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S., Chemoselective metal-free aerobic alcohol oxidation in lignin. Journal of the American Chemical Society 2013, 135, 6415-6418; and Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S., Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature 2014, 515, 249-252.
As this subfield of lignin valorization continues to mature, some promising homogeneous catalytic oxidation methods have emerged. Hanson et al. developed a series of oxovanadium catalysts featuring various ligands and successfully applied them to the oxidative cleavage of lignin. Depending on which catalyst they employed, the researchers were able to selectively cleave C—C or C—O bonds in phenolic lignin model compounds. Sedai, B.; Diaz-Urrutia, C.; Baker, R. T.; Wu, R. L.; Silks, L. A.; Hanson, S. K., Comparison of copper and vanadium homogeneous catalysts for aerobic oxidation of lignin models. ACS Catalysis 2011, 1, 794-804; Hanson, S. K.; Baker, R. T., Knocking on wood: base metal complexes as catalysts for selective oxidation of lignin models and extracts. Accounts of Chemical Research 2015, 48, 2037-2048; Hanson, S. K.; Wu, R. L.; Silks, L. A., C—C or C—O Bond cleavage in a phenolic lignin model compound: selectivity depends on vanadium catalyst. Angewandte Chemie—International Edition 2012, 51, 3410-3413; and Sedai, B.; Diaz-Urrutia, C.; Baker, R. T.; Wu, R. L.; Silks, L. A.; Hanson, S. K., Aerobic oxidation of β-1 lignin model compounds with copper and oxovanadium catalysts. ACS Catalysis 2013, 3, 3111-3122.
Mottweiler successfully used vanadium acetylacetonate and copper (II) nitrate as catalysts to cleave lignin model compounds and generate carboxylic acids in modest yields. Mottweiler, J.; Puche, M.; Rauber, C.; Schmidt, T.; Concepcion, P.; Corma, A.; Bolm, C., Copper- and vanadium-catalyzed oxidative cleavage of lignin using dioxygen. Chemsuschem 2015, 8, 2106-2113.
Bozell and coworkers improved the performance of cobalt-Schiff base complexes in the oxidation of lignin model compounds. Biannic, B.; Bozell, J. J., Efficient cobalt-catalyzed oxidative conversion of lignin models to benzoquinones. Organic Letters 2013, 15, 2730-2733. They demonstrated that Co(salen) complexes can selectively convert phenolic lignin models into benzoquinones in good yields. Due to low abundance of free phenolic moieties in lignin, however, this method generates these benzoquinones from isolated lignins in only very low yields.
Each of these catalytic oxidation methods, in which O2 was used as the ultimate oxidant, offers unique opportunities for selective chemical synthesis depending on the transition metal and ligand scaffold. However, they are all limited by the long reaction times required even for the simplest of lignin model compounds. Efficient catalytic conversion of both lignin model compounds and lignin to well-defined aromatics represents a key challenge that has considerably limited the valorization of lignin. To date, this challenge remains long-felt and unmet.
Previously, dioxomolybdenum compounds have been reported as efficient catalysts for the oxidation of alcohols. See Jeyakumar, K.; Chand, D. K., Aerobic oxidation of benzyl alcohols by Mo—VI compounds. Applied Organometallic Chemistry 2006, 20, 840-844; Jeyakumar, K.; Chand, D. K., Application of molybdenum(VI) dichloride dioxide (MoO2Cl2) in organic transformations. Journal of Chemical Sciences 2009, 121, 111-123; Enemark, J. H.; Cooney, J. J. A., Synthetic analogues and reaction systems relevant to the molybdenum and tungsten oxotransferases. Chemical Reviews 2004, 104, 1175-1200; Chen, C. T.; Kuo, J. H.; Ku, C. H.; Weng, S. S.; Liu, C. Y., Nucleophilic acyl substitutions of esters with protic nucleophiles mediated by amphoteric, oxotitanium, and vanadyl species. Journal of Organic Chemistry 2005, 70, 1328-1339; Sanz, R.; Escribano, J.; Aguado, R.; Pedrosa, M. R.; Anaiz, F. J., Selective deoxygenation of sulfoxides to sulfides with phosphites catalyzed by dichlorodioxomolybdenum(VI). Synthesis—Stuttgart 2004, 1629-1632; and Sanz, R.; Aguado, R.; Pedrosa, M. R.; Arnaiz, F. J., Simple and selective oxidation of thiols to disulfides with dimethylsulfoxide catalyzed by dichlorodioxomolybdenum(VI). Synthesis—Stuttgart 2002, 856-858. But, dioxomolybdenum compounds have not been used on lignin or even simple lignin model compounds. Recently, Sanz and coworkers reported on the oxidative cleavage of glycols catalyzed by a common and easily prepared dioxomolybdenum (VI) complex in dimethyl sulfoxide (DMSO) using microwave irradiation. See Garcia, N.; Rubio-Presa, R.; Garcia-Garcia, P.; Fernandez-Rodriguez, M. A.; Pedrosa, M. R.; Arnaiz, F. J.; Sanz, R., A selective, efficient and environmentally friendly method for the oxidative cleavage of glycols. Green Chemistry 2016, 18, 2335-2340. As an efficient and direct manner of heating the reaction mixture rapidly, microwave irradiation has already been successfully explored in organic synthesis and lignin pretreatments. See, for example, Conesa, T. D.; Campelo, J. M.; Clark, J. H.; Luque, R.; Macquarrie, D. J.; Romero, A. A., A microwave approach to the selective synthesis of omega-laurolactam. Green Chemistry 2007, 9, 1109-1113; Badamali, S. K.; Luque, R.; Clark, J. H.; Breeden, S. W., Microwave assisted oxidation of a lignin model phenolic monomer using Co(salen)/SBA-15. Catalysis Communications 2009, 10, 1010-1013; and Sutradhar, M.; Alegria, E. C. B. A.; Mahmudov, K. T.; Silva, M. F. t. C. G. d.; Pombeiro, A. J. L., Iron(III) and cobalt(III) complexes with both tautomeric (keto and enol) forms of aroylhydrazone ligands: catalysts for the microwave assisted oxidation of alcohols. RSC Advances 2016, 6, 8079-8088.